Disproportionation of saturated hydrocarbons employing a catalyst that comprises platinum and tungsten

ABSTRACT

A process for disproportionation of saturated hydrocarbons which comprises contacting the saturated hydrocarbons in a disproportionation reaction zone and in the presence of no more than 5 weight percent olefins with a catalyst mass comprising a platinum component and a tungsten component, wherein the volumetric ratio of the platinum component to the tungsten component is greater than 2:7 and less than 7:2. Preferably the volumetric ratio of the platinum component to the tungsten component is between 3:7 and 10:7, and the reaction is preferably carried out at a temperature between 400* to 800*F.

Burnett 1 DllSTlRDPURTTDNATllON OF SATURATED HYDROCARDDNS EMPLOYTNG A CATALYST THAT COMPRTSES PLATINUM AND TUNCSTEN [75] Inventor: Robert L. Burnett, Pinole, Calif.

[73] Assignee: Chevron Research Company, San

Francisco, Calif.

[22] Filed: Feb. 8, 1973 [21} Appl. No.: 330,534

Related US. Application Data [63] Continuation of Ser. No. 108,377, Jan. 21, 1971.

[52] US. Cl. 260/676 R, 260/683 D [51] Hnt. Cl. C07c 9/00, C070 3/00 [58] Field of Search 260/676 R, 683 D [56] References Cited UNITED STATES PATENTS 3,431,316 3/1969 Banks 260/683 D 3,445,541 5/1970 Heckelsberg et a1. 260/683 D 3,484,499 12/1969 Lester et a1 260/673 3,668,268 6/1972 Mulaskey 260/676 R [111 Dec. 24, 11974 3,718,706 2/1973 Sieg 260/683 D 3,773,845 11/1973 Hughes 260/676 R 3,773,850 11/1973 Tischler et a1. 260/677 R 3,775,505 11/1973 Hughes 260/676 R [57] ABSTRACT A process for disproportionation of saturated hydrocarbons which comprises contacting the saturated hydrocarbons in a disproportionation reaction zone and in the presence of no more than 5 weight percent olefins with a catalyst mass comprising a platinum component and a tungsten component, wherein the volumetric ratio of the platinum component to the tungsten component is greater than 2:7 and less than 7:2. Preferably the volumetric ratio of the platinum component to the tungsten component is between 3:7 and 10:7, and the reaction is preferably carried out at a temperature between 400 to 800F.

6 Claims, 1 Drawing Figure 7o CONVERSION PATENTEU 318569876 0 2o 40 so so 100 120 HOURS UNVENTOR ROBERT L. BURNETT ATTORNEYS DISIPROPORTIONATION OlF SATURATED IHIYDROCARBONS EMPLOYING A CATALYST THAT COMPRISES PLATINUM AND TUNGSTEN CROSS REFERENCE TO RELATED APPLlCATlONS This is a continuation, of application Ser. No. 108,377, filed Jan. 21, 1971.

This application is related to commonly assigned applications Serial Nos. 3,303 now US. Pat. Nos. 3,775,505, 3,306 now U.S. Pat. No. 3,773,845 and 22,791, now abandoned the disclosures of which patent applications are incorporated by reference into the present specification, particularly those portions of the afore-identified applications directed to disproportionation using a catalyst mass comprising platinum and tungsten components.

BACKGROUND OF THE INVENTION l. Field of the Invention The present invention relates to the conversion of saturated hydrocarbon feeds to hydrocarbon products with different distributions of molecular weights than those of the feeds. More particularly, the present invention relates to disproportionation of saturated hydrocarbons.

The term disproportionation is used herein to mean the conversion ofhydrocarbons to new hydrocarbons of both higher and lower molecular weight. For example, butane may be disproportionated according to the reaction:

Saturated hydrocarbon as used herein includes hydrocarbon molecules which are completely saturated with hydrogen and/or hydrocarbon molecules which are partially saturated with hydrogen but contain at least one alkyl group which is completely saturated with hydrogen. Thus, the term saturated hydrocarbon" as used herein includes alkanes (paraffins); alicyclics (cyclo-paraffins); branched-chain alkanes; alicyclic hydrocarbons with one or more attached alkane groups; and unsaturated hydrocarbons with one or more attached, completely saturated hydrocarbon groups, as, for example, an aromatic hydrocarbon with an attached alkane. From the description hereinbelow, it will become apparent that in the instance of unsaturated hydrocarbons with an attached, completely saturated hydrocarbon group the conversion process of the present invention operates by way of the completely saturated hydrocarbon group.

2. Prior Art A number of processes have been disclosed for converting various hydrocarbons to higher molecular weight hydrocarbons. For example, polymerization has been proposed for increasing the molecular weight of hydrocarbons such as gaseous, or low-boiling olefins. Various processes for olefin polymerization have been disclosed, including processes wherein the polymerization reaction is'catalyzed with inorganic acids such as sulfuric or phosphoric.

' To obtain the olefinic feed for a polymerization reaction,'both thermal cracking and catalytic dehydrogenation processes have been proposed. For example, a two-stage process has been proposed wherein hydrocarbon gases are first cracked to form substantial amounts of olefins. Then the olefins are polymerized to higher-boiling compounds by contacting the olefins with a catalyst adapted to promote the forming of heavier hydrocarbons by polymerization.

U.S. Pat. No. 1,687,890 is directed to a process of converting low-boiling point hydrocarbons into higherboiling point hydrocarbons by mixing a hydrocarbon vapor with steam and then contacting the steamhydrocarbon mixture with iron oxide at temperatures in excess of 1,112F. It is theorized in U.S. Pat. No. 1,687,890 that the following reactions may be involved to a greater or lesser extent:

1. Paraffin hydrocarbons on being brought into contact with ferric oxide at elevated temperatures are oxidized or dehydrogenated, forming unsaturated hydrocarbons.

2. Unsaturated hydrocarbons of low molecular weight polymerize into unsaturated hydrocarbons of higher molecular weight when subjected to ele vated temperatures, the extent of polymerization depending upon the temperature and duration of treatment.

7. Unsaturated hydrocarbons are hydrogenated by nascent hydrogen."

Another process which has been proposed for converting hydrocarbons to higher molecular weight hydrocarbons is olefin disproportionation. Numerous methods and catalysts have been disclosed for the disproportionation of olefins. In most of these processes, the olefin is disproportionated by contacting with a catalyst such as tungsten oxide or molybdenum oxide on silica or alumina at a temperature between about and 1100F and at a pressure between about 15 and 1,500 psia. These prior art processes have been directed to an effective method to convert essentially only olefins, not saturated hydrocarbons, to higher molecular weight hydrocarbons by disproportionation.

For example, in U.S. Pat. No. 3,431,316, an olefin disproportionation process is disclosed, and it is stated that, if desired, paraffinic and cycloparaffinic hydrocarbons having up to 12 carbon atoms per molecule can be employed as diluents for the reaction; that is, the saturated hydrocarbons are nonreactive and merely dilute the olefins which are the reactants.

A process for the direct conversion of saturated hydrocarbons to higher molecular weight hydrocarbons would be very attractive because in many instances saturated hydrocarbons are available as a relatively cheap feedstock. For example, in manyinstances, excess amounts of propane and/or butanes are available in an over-all refinery operation.

Processes which have been previously reported wherein saturated hydrocarbons are disproportionated include contact of saturated hydrocarbons with solid catalyst comprised of A1Cl on A1 0 and contact of saturated hydrocarbons with a promoter comprised of alkyl fluoride and BE. The use of the AlCl solid catalyst was uneconomic because, among other reasons, the catalyst was nonregenerable. The use of alkyl fluoride and BP was unattractive because of severe corrosion, sludge formation and other operating problems.

In the past it has been the practice to convert saturated hydrocarbons, particularly normal alkanes, to olefins as a separate or distinct step and then to disproportionate the olefins to valuable higher molecular weight hydrocarbons.

For example, in U.S. Pat. No. 3,431,316, saturated light hydrocarbons are cracked to form olefins, and then the olefins are separated from the cracker effluent and fed to a disproportionation zone wherein the olefins are disproportionated to higher molecular weight hydrocarbons. Thus, a separate step is used to obtain olefins, because, according to the prior art, no economically feasible process is available for the direct disproportionation of saturated hydrocarbons.

U.S. Pat. No. 3,445,541 discloses a process for the dehydrogenation-disproportionation of olefins and paraffins, using a combined dehydrogenation and disproportionation catalyst. According to U.S. Pat. No. 3,445,541, a hydrocarbon feed which is either an acyclic paraffin or acyclic olefin having three-six carbon atoms is contacted with the catalyst at conditions of temperature and pressure to promote dehydrogenation and disproportionation. It is said that the process can be carried out at temperatures between 800F and 1,200F; however, the lowest temperature used for processing a paraffin in accordance with any of the examples of U.S. Pat. No. 3,445,541 is 980F, and typically the temperature used is between l,040F and 1,125F.

The high temperature process disclosed in U.S. Pat. No. 3,445,541 is shown therein to result in only relatively low yields of saturated higher molecular weight hydrocarbons. The U.S. Pat. No. 3,445,541 process operates with a substantial amount of olefins in the reaction zone and thus with about 10 to 50 volume percent or more olefins in the effluent from the disproportionation reaction zone.

SUMMARY OF THE INVENTION According to the present invention a process is provided wherein saturated hydrocarbons are disproportionated, which process comprises contacting the saturated hydrocarbons in a disproportionation reaction zone and in the presence of no more than 5 weight percent olefins with a catalyst mass comprising a platinum component and a tungsten component, wherein the volumetric ratio of the platinum component to the tungsten component is greater than 2:7 and less than 7:2. Preferably the volumetric ratio of the platinum component to the tungsten component is between 3:7 and 10:7, and the reaction is preferably carried out at a temperature between 400 to 800F.

The present invention is based, among other factors, on the unexpected finding that the fouling rate of saturated hydrocarbon disproportionation catalyst masses comprising a platinum component and a tungsten component is considerably reduced when going from a catalyst mass containing 2 parts by volume of a platinum component to 7 parts by volume of a tungsten component to a catalyst mass which is the same except has a higher volumetric ratio of the platinum component to the tungsten component. For example, I have found that in changing the volumetric ratio of platinum to tungsten from 2:7 to 4:7 the fouling rate of the catalyst was more than out in half for alkane disproportionation. As discussed in earlier commonly assigned application Ser. No. 3,303 now U.S. Pat. No. 3,775,505, we have found that it is undesirable to have considerable amounts of olefin present in the reaction zone and that the addition of olefin to the feedstock to the reaction zone will tend to kill the disproportionation reactions desired in accordance with the process of the present invention. This was a surprising finding, especially in In my experimental work directed particularly to lowering the fouling rate of the disproportionation catalysts, l have found that it is extremely important to maintain the olefin concentration in the disproportionation reaction zone below about 5 weight percent and more preferably below about 1.0 weight percent. Thus the olefins in the effluent from the disproportionation reaction zone should be below 5.0 weight percent and more preferably below 1.0 weight percent. Although I have found that catalyst masses of increased platinum componentsztungsten components volumetric ratio (and hence increased catalytic surface area ratio) reduce the fouling rate at a given olefin concentration in the disproportionation reaction zone, I have also found that it is critical to still maintain a relatively low olefin concentration in the reaction zone in order to maintain a relatively low fouling rate for the disproportionation catalyst even at the increased volumetric ratio for the platinum component: tungsten component.

Disproportionation catalyst masses of increased stability, that is, lower fouling rate, in accordance with the present invention have been found to be accompanied by a lower olefin fraction formed in the disproportionation reaction. A possible explanation for the increased stability of the disproportionation catalyst masses in accordance with the present invention may be as follows: olefins produced on platinum sites by dehydrogenation are very readily disproportionated on the extremely active tungsten sites. These disproportionated products are then rehydrogenated to open up more catalyst sites for subsequent disproportionation reactions. As the platinum hydrogenation activity declines, the olefin concentration builds up if the olefins are not reacted away as fast as they were reacted away before the platinum hydrogenation activity declined. Increased volumetric amounts of platinum component result in an increased amount of platinum catalyst sites so that the limitation for the catalyst mass in terms of fouling rate, that is, decrease in conversion per unit of time, is not as fast because the catalyst has more ability to cope with hydrogenation of olefins.

It has also been theorized that the olefins are detrimental to the disproportionation reactions of the present invention because the olefins adsorb relatively strongly onto the hydrogenation-dehydrogenation catalytic sites and thus prevent the saturated hydrocarbon feedstock molecules from reaching these catalytic sites. In our laboratory work, we have noted that after the saturated hydrocarbon disproportionation has been substantially inhibited by the injection of olefins to the disproportionation reaction zone, most of the catalytic activity can be recovered by discontinuing the olefin injection to the reaction zone.

Other theories may be postulated as to why the presence of more than a few weight percent olefins in the reaction zone poisons the saturated hydrocarbon disproportionation reaction of the present invention. For

example, certain work by Wood, reported on page 30, Vol. ll of the Journal of Catalysis (1968), indicates that the presence of adsorbed hydrogen is necessary for cyclohexane dehydrogenation to occur. This adsorbed hydrogen may be selectively scavenged by substantial quantities of olefins present in the disproportionation reaction zone, thereby preventing the saturated hydrocarbon feedstock molecules from being dehydrogenated.

Whether because of a combination of the above theories, or one of the theories separately, or some other theory, the finding remains that low fouling rate advantages of the process of the present invention requires that the concentration of olefins in the disproportionation reaction zone be maintained at a low level.

It is also preferred in the process of the present invention to operate the reaction zone at a pressure above at least 100 psig. The elevated pressure has been found advantageous because it leads to higher disproportionation conversion. The residence time of the reactant in the reaction zone increases with increasing pressure. Also, the equilibrium partial pressures of both olefin and H formed from dehydrogenation of saturated hydrocarbons rise in direct proportion to the square root of the total pressure. The equilibrium concentrations of olefins, relative to those of the saturated hydrocarbons from which they are formed, are inversely proportional to the square root of the total pressure. Thus, relatively high pressures, of the order of.5002,000 psig, are particularly preferred in the process of the present invention as the higher pressures improved catalyst stability.

While the process of the present invention requires that no more than 5 weight percent olefins be present in the reaction zone, it is preferred to maintain the olefin concentration still lower as, for example, below about 2 weight percent olefins, and still more preferably, below about 1 weight percent olefins. To maintain the olefin concentration at a relatively low level, various means may be employed. Temperatures below about 800F and elevated pressures above at least 100 psig are particularly desirable to maintain the olefin concentration at a relatively low level in the disproportionation reaction zone. In accordance with one particularly preferred embodiment of the present invention, the temperature in the disproportionation reaction zone is maintained below. about 800F, the pressure is maintained above at least 100 psig, and the olefin concentration is maintained below about 0.5 weight percent.

Although it is advantageous to maintain the temperature in the reaction zone below about-850F and more preferably below about 800F in order to maintain relatively low olefin concentration, it is also particularly important to maintain the temperature below about 850F, and more preferably below about 800F in order to obtain a relatively high yield of saturated hydrocarbons which are of higher molecular weight than the feed-saturatedhydrocarbons. Thus, for example, when butanes or propane are fed to the disproportionation reaction zone, a much better ultimate yield of C material is obtained when operating at the relatively low temperatures. Temperatures in the range of 500 to 700F are particularly desirable.

Still further. the relatively low temperatures, particularly below 800F, have been found by us to be extremely advantageous from the standpoint of catalyst stability. That is, the fouling rates of catalysts used in the process of the present invention have been found to be considerably lower when operating at the relatively low temperatures, i.e., below about 800F, as opposed to operating temperatures above 800F and particularly operating temperatures above 850F.

In our commonly assigned application Ser. No. 22,791 now abandoned, the increased stability for a catalyst mass containing a tungsten component having less than 8 wt. percent W, for example about 2 percent W by weight, is discussed. In the process of the present invention we have found that the stability or the low tungsten-content catalysts are still further improved, by a factor of more than 200 percent in most instances, by using platinum componentztungsten component ratios in the disproportionation catalyst mass above 2 parts platinum to 7 parts tungsten. Preferably the volumetric ratio ofthe platinum component ot the tungsten component is greater than 2:7 but less than 7:2. More pref erably, the volumetric ratio of the platinum component to the tungsten component is between 3:7 and l0:7.

Preferably the amount of platinum by weight contained in the platinum component is between 0.1 and 5 wt. percent Pt and preferably the amount of tungsten contained in the tungsten component is between 0.5 and 15 wt. percent W. The platinum can be combined with or carried on various supports, but alumina is particularly preferred. In most instances it is preferred to combine a small amount of lithium with the platinum component in order to reduce the acidity of the platinum-alumina component to a minimum.

The tungsten component can also be carried on a variety of supports. However, it is particularly preferred to have the tungsten supported on silica, and it is particularly preferred to use the tungsten in the form of tungsten oxide. I have found that particularly preferred amounts of tungsten are between 1.0 and 4.0 wt. percent W in the tungsten component to cooperate with the relatively high volumetric ratio of platinum component to tungsten component in achieving a high stability for the disproportionation catalyst in the process of the present invention.

In the preferred embodiment of the present invention, high stability, or low fouling rate, in the disproportionation process is achieved by the cooperation of 5 features in the process of the present invention. First, the disproportionation catalyst mass must have a relatively high Pt:W component volumetric ratio, preferably between 3 parts Pt to 7 parts W and 10 parts Pt to 7 parts W. Secondly, the disproportionation catalyst mass should have a tungsten component with a weight percent of tungsten below about 8 wt. percent W, pref erably between about 1.0 and 4.0 wt. percent W. Thirdly, the temperature for the disproportionation reaction should be below 850F. and more preferably below 800F. Fourthly, the percent olefins in the disproportionation reaction zone should be below 5 wt. percent, and more preferably below 1.0 wt. percent. Fifthly, the pressure should be above psig, more preferably above 500 psig, in the reaction zone.

EXAMPLES AND DESCRIPTION OF THE DRAWING l. The drawing shows the results of a first set of comparative runs illustrating the increased stability obtained in accordance with the process of the present invention. The data upon which FIG. I is based were obtained by contacting normal butane with a saturated hydrocarbon disproportionation catalyst mass under the following conditions:

Volume of Catalyst in Reactor: 9 cubic centimeters (cc.)

Type of Catalyst:

Run 1 2 cc. of 0.5 wt. Pt; 0.5 wt. Li on A1 7 cc. of 2.5 wt. WO (calc. as W0 on SiO Run 2 Same catalyst components as in Run 1 except that 4 parts by volume of the platinum component were used with 7 parts by volume of the tungsten component. Operating conditions and feed werev the same as Run 1.

Both types of catalyst particles were 42 to 60 Tyler mesh size.

(The volumetric ratios required in the process of the present invention are based on the respective catalyst components having about the same particle size or particle size distribution.)

Operating Conditions:

Temperature: 750F.

Pressure: 900 psig Feed Rate: 9 cc./hour As can be seen by the comparison of Run 1 with Run 2, graphically depicted in the drawing, a considerably higher stability was achieved in Run 2 than in Run 1. Run 2 was carried out in accordance with the process of the present invention using a catalyst mass having a ratio of platinum component to tungsten component greater than 2:7. In particular, the ratio of the platinum component to the tungsten component was 4:7. The fouling rate using the largeramount of platinum in accordance with the processof the present invention was reduced by a factor of almost 300 percent. The fouling rate was 0.0030 for Run 1 as compared to 0.0011 for Run 2.

2. A second set of comparative runs was made for two disproportionation catalyst masses of differing volumetric ratios of platinum componentztungsten component. The reaction conditions used in the second set of 40 runs were as follows:

Run 3 Feed: Normal butane Volume of Catalyst in Reactor: 9 cc.

Type of Catalyst:

2 cc. of 0.5 wt. Pt; 0.5 wt. Li on A1 0 7 cc. of 2.0 wt. W0 (calc. as W0 on SiO Both types of catalyst particles were 42 to 60 Tyler mesh size.

Operating conditions were the same as Run 1.

Run 4 Volume of Catalyst in Reactor: 9 cc.

Type of Catalyst:

2 cc. of 0.5 wt. Pt; 0.5 wt. Li on M 0 5 cc. of 2.0 wt. W0 (calc. as W0 on SiO Both types of catalyst particles were 42 to 60 Tyler mesh size.

Operating conditions were the same as Run 1.

In this set of comparative runs the fouling rate using the catalyst mass having 2 parts platinum component by volume to 7 parts tungsten component was 0.00127, whereas the fouling rate for Run 4, having 4 parts platinum component by volume to 5 parts tungsten component, was only 0.00059, thus again resulting in a dramatic and unexpected decrease in the fouling rate for the disproportionation catalyst mass by increasing the amount, of the platinum component relative to the tungsten component.

3. A fifth test run was made using a relatively high volumetric ratio of platinum component to tungsten component under the following reaction conditions:

Run 5 Feed: normal butane Volume ofCatalyst in Reactor: 9 cc.

Type of Catalyst:

7 cc. of 0.5 wt. Pt; 0.5 wt. Li on Al O 2 cc. of 2.0 wt. W0 (calc. as W0 on SiO Both catalyst particles were 28 to 60 Tyler mesh size. Operating conditions were the same as Run 1. Using this catalyst mass, the fouling rate was 0.0013.

compared to the approximately same fouling rate ol 0.0012 for 2 parts platinum component: 7 parts tungsten component in Run 3 above. It is seen that increasing the platinum component beyond about 7 parts platinum to 2 parts tungsten does not result in an improvement of catalyst compared to the base case of about 2 parts tungsten component to 7 parts platinum component.

Although various embodiments of the invention have been described, it is to be understood that they are meant to be illustrative only and not limiting. Certain features may be changed without departing from the spirit or scope of the invention. It is apparent that the present invention has broad application to the disproportionation of saturated hydrocarbons using a catalyst mass having an increased volumetric ratio of platinum component totungsten component and preferably having the volumetric ratio of platinum component to tungsten component greater that 2:7 and less than about 7:2. Accordingly, the invention is not to be con strued as limited to the specific embodiments or examples discussed but only as defined in the appended claims or substantial equivalents thereto.

What is claimed is:

l. A process for disproportionation of saturated hydrocarbons which comprises contacting the saturated hydrocarbons in a disproportionation reaction zone at a temperature between 400F and below 800F and a pressure between and 2,000 psig and in the presence of no more than 5 weight percent olefms in said reaction zone with a catalyst mass comprising a platinum component and a tungsten component wherein the volumetric ratio of the platinum component to the tungsten component is greater than 2:7 and less than 7:2.

2. A process in accordance with claim 1 wherein the weight percent of platinum in the platinum component is between 0.1 and 5.0 weight percent platinum and wherein the weight percent tungsten in the tungsten component is between 0.5 and 15 weight percent tungsten.

3. A process in accordance with claim 1 wherein the tungsten component comprises tungsten oxide on silma.

4. A process in accordance with claim 1 wherein the weight percent tungsten in the tungsten component is between 1.0 and 4.0.

5. A process for the disproportionation of alkanes which comprises contacting the alkanes at a temperature between 400 and below 800F. and a pressure between 100 and 2,000 psig with a catalyst mass comprising a platinum component containing 0.1 to 5.0 weight percent platinum on an alumina support and a tungsten component comprising tungsten oxide on silica and containing between 0.5 weight percent tungsten and 15 weight percent tungsten, and wherein the volumetric ratio of the platinum component to the tungsten component is greater than 2:7 and less than 7:2.

6. A process in accordance with claim 5 wherein the volumetric ratio of the platinum component to the tungsten component is between about 3:7 and i027.

* a t a: 

1. A PROCESS FOR DISPROPORTIONATION OF SATURATED HYDROCARBONDS WHICH CONPRISES CONTACTING THE SATURATED HYDROCARBONS IN A DISPROPORTIONATION REACTION ZONE AT A TEMPERATURE BETWEEN 400*F AND BELOW 800*F AND A PRESSURE BETWEEN 100 AND 2,000 PSIG AND IN THE PRESENCE OF NO MORE THAN 5 WEIGHT PERCENT OLEFINS IN SAID REACTION ZONE WITH A CATALYST MASS COMPRISING A PLATINUM COMPONENT AND A TUNGSTEN COMPONENT WHREIN THE VOLUMETRIC RATIO OF THE PLATINUM COMPONENT TO THE TUNGSTEN COMPONENT IS GREATER THAN 2:7 AND LESS THAN 7:2.
 2. A process in accordance with claim 1 wherein the weight percent of platinum in the platinum component is between 0.1 and 5.0 weight percent platinum and wherein the weight percent tungsten in the tungsten component is between 0.5 and 15 weight percent tungsten.
 3. A process in accordance with claim 1 wherein the tungsten component comprises tungsten oxide on silica.
 4. A process in accordance with claim 1 wherein the weight percent tungsten in the tungsten component is between 1.0 and 4.0.
 5. A process for the disproportionation of alkanes which comprises contacting the alkanes at a temperature between 400* and below 800*F. and a pressure between 100 and 2,000 psig with a catalyst mass comprising a platinum component containing 0.1 to 5.0 weight percent platinum on an alumina support and a tungsten component comprising tungsten oxide on silica and containing between 0.5 weight percent tungsten and 15 weight percent tungsten, and wherein the volumetric ratio of the platinum component to the tungsten component is greater than 2:7 and less than 7:2.
 6. A process in accordance with claim 5 wherein the volumetric ratio of the platinum component to the tungsten component is between about 3:7 and 10:7. 